Facsimile apparatus is used for the digital electrical transmission of documents over long distances. Typically, a document is scanned line-by-line to generate a digitized representation of each scanned line of the document. Each bit of information, either a ZERO or ONE corresponds to a small area of the document. The color of the area, i.e., whether it is predominantly white or black, determines whether the associated binary signal is a ZERO or a ONE, respectively.
Transmitting a binary signal for each small area of the document requires a large amount of digital information and transmitting one 81/2 by 11 inch page can take as long as fifteen minutes. Therefore a variety of techniques have been proposed to reduce the amount of information used to represent a scanned line of a document. For a general explanation of such techniques, see the article "International Digital Facsimile Coding Standards", by R. Hunter and A. H. Robinson in the Proceedings of the Institute of Electrical and Electronic Engineers, Volume 68, No. 7, July 1980, pages 854-867. This article describes, in particular, a modified two-dimensional relative element address designate (READ) coding procedure as defined in Recommendations T.4 (Group 3) and T.6 (Group 4) of the Commite Consultative International pour Telephonie et Telegraphie (CCITT). For a complete description of this coding procedure reference should be had to U.S. Pat. No. 4,562,484 "A Method and Device for Decoding Two-Dimensional Facsimile Signals", assigned to the assignee of the instant application, and U.S. Pat. No. 4,558,371, entitled "Method and Device for Two-Dimensional Facsimile Coding", assigned to the assignee of the instant application; both patents hereby incorporated by reference.
Generally speaking, two-dimensional coding employs statistically-developed correlations of information in the vertical direction of documents to be transmitted. In this manner, commonly-occurring inter-line patterns can be encoded in a way which significantly reduces the amount of information used to represent a scanned line of the document. Information regarding a reference line, which has already been processed, is used to code the information on a subsequent scanned line, called the coding line.
The previously-mentioned patents disclose methods and devices for performing such two-dimensionl coding and decoding which offer many advantages over the prior art. However, as those methods and devices require that the information regarding the reference line be reaccessed from a so-called "picture memory" during the encoding of the coding line, in order to compare their differences, the coding and decoding process is slowed considerably due to the resulting doubling of information which must be transferred on the bus from the picture memory.
Another aspect of the methods and devices described in the co-pending applications, mentioned above, is their reliance on so-called "end-of-line" codes to signify the end of transmission or reception of a line. Such codes contain no inherent document-specific information, and simply represent an appreciable overhead cost added to each line of the document.
Yet another aspect of the methods and devices described in the co-pending applications, is their calculation of accumulated run-lengths during the processing of so-called "uni-color" line, i.e., those which are encoded as all ONE's or all ZERO's. This calculation imposes an overhead cost during the processing of what are otherwise very easily encoded or decoded scanned lines. Since statistics reveal that a considerable portion of scan lines in ordinary documents do not have color changes in them, for example, a simple business letter can be expected to contain 60% all white scan lines, any of the three overhead costs mentioned in connection with the processing of lines by the prior art represent substantial slow downs in the processing of documents by fascimile apparatus.